The motor of a submersible motor-pump combination is usually immersed in a lubricating fluid, such as oil, which is sealed from the well fluid, for example, an oil-brine mixture in oil wells or geothermal water wells. The motor lubricating fluid generally attains a temperature in excess of that in the well, the heat being generated by the motor friction, windage and copper core losses. The generated heat is internally distributed by the motor fluid and is transferred via the motor housing to the well fluid which is at a lower temperature. This basic conduction transfer of heat to the well fluid is inefficient.
Typically, a 100 HP motor will generate about 14 KW of waste heat which must be removed to avoid motor overheating and potential motor burn-out.
In the absence of heat extracting means, motors operate in wells at temperatures up to about 100.degree. F. above that of the well fluid. When the latter is about 150.degree. F., motor burn-out and overheating is not a problem. However, when the well fluid is about or in excess of 300.degree. F. the usual heat transfer by conduction through the motor housing wall may not be satisfactory to avoid motor overheating and possible early burn-out.